Thin film electroluminescence display device and method of manufacturing the same

ABSTRACT

An electroluminescence display device including a substrate, a corrugated structure formed on the substrate, wherein the corrugated structure disperses light through diffraction and reflection; and a first electrode layer, a first insulation layer, a fluorescent layer, a second insulation layer, and a second electrode layer sequentially formed on the substrate to follow the shape of the corrugated structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the divisional of U.S. patent application Ser. No.10/715,416, filed Nov. 19, 2003, now allowed, in the U.S. Patent andTrademark Office, which claims the priority of Korean Patent ApplicationNo. 2003-9094, filed on Feb. 13, 2003, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an electroluminescence display deviceand a method of manufacturing the same, and, more particularly, to anelectroluminescence display device which is provided with an improvedsubstrate and thin-film layers, thereby achieving an improvement inlight output efficiency depending on the refractive index of each thinfilm, and a method of manufacturing the same. 2. Description of theRelated Art

Generally, electroluminescence display devices are high-level imagedisplay devices. They are flat display devices having many advantagessuch as a small size, a light weight, environmental resistance,durability, a long life span, and a wide viewing angle.

In electroluminescence display devices, when a voltage is applied toboth ends of a fluorescent layer, electrons are accelerated toward theinside of the fluorescent screen so as to collide with an atom, i.e., aluminescence center. Thus, the electrons at the electron level of theatom are excited to a higher energy level, and then make a transition toa ground state. At this time, due to an energy gap around the electrons,light having a particular wavelength range is produced, that is, anelectroluminescence phenomenon occurs.

Electroluminescence display devices are divided into alternating current(AC) drive types and direct current (DC) drive types, or thin-film typesand thick-film types. Such electroluminescence display devices usuallyhave at least one insulation layer and at least one fluorescent layer,and are provided with an electrode, which applies a voltage to both endsof the fluorescent layer. In order to improve the characteristics of theinsulation layer and the fluorescent layer, these layers can be formedin a multilayer structure having a plurality of thin films made ofdifferent materials.

FIG. 1 shows an example of an AC drive type thin-filmelectroluminescence display device. Referring to FIG. 1, a firstelectrode layer 11, a first insulation layer 12, a fluorescent layer 13,a second insulation layer 14, and a second electrode layer 15 aresequentially formed on a substrate 10. The fluorescent layer 13 can bemade of a metal sulfide such as ZnS, SrS, or CsS, an alkaline-earthpotassium sulfide such as CaCa₂S₄ or SrCa₂S₄, or a metal oxide. For theatoms, i.e., luminescence centers, contained in a material of thefluorescent layer 13, transition metals containing Mn, Ce, Tb, Eu, Tm,Er, Pr, or Pb or alkaline ash metals can be used.

Another example of an electroluminescence display device is disclosed inJapanese Patent Publication No. 2001-176671. This electroluminescencedisplay device has a structure in which a first electrode, an insulationlayer of an inorganic compound, a luminescence layer of an inorganiccompound, and a second electrode are stacked on a substrate.

In the electroluminescence display device shown in FIG. 1, when apredetermined voltage is applied to the first and second electrodelayers 11 and 15 located at both sides of the fluorescent layer 13,light having a particular wavelength is produced due to anelectroluminescence phenomenon.

In such an electroluminescence display device, a substantially flat thinfilm forms the top surface of each of the fluorescent layer 13 and thefirst and second electrode layers 11 and 15, and the refractive index ofthe first and second electrode layers 11 and 15 is high, so most of thelight produced in the fluorescent layer 13 is not transmitted throughthe fluorescent layer 13 and the second electrode layer 15. Accordingly,only about 10% of the light is radiated from the electroluminescencedisplay device.

More specifically, light efficiency of an electroluminescence displaydevice is divided into internal efficiency and external efficiency.While the internal efficiency depends on the characteristics of thefluorescent layer, i.e., luminescence material, the external efficiencydepends on the refractive index of each layer constituting the displaydevice. The external efficiency η_(ex) can be expressed asη_(ex)=η_(in)×η_(out). Here, η_(in) denotes the internal efficiency, andη_(out) denotes output coupling efficiency. A major restriction on theoutput coupling efficiency in a thin film electroluminescence display isrelated to the extraction of light generated inside the device to theexternal environment. The large mismatch between the refractive index ofthe thin film phosphor and air results in a large proportion of thelight rays undergoing total internal reflection. Some of the lightgenerated inside the thin film phosphor thus becomes trapped, unable toescape into the air. This effect plagues EL structures employing thethin film phosphor. According to Snell's law, only light emitted at anangle less than the critical angle can escape from the surface, allother light is internally reflected back into the device. A fluorescentlayer made of a material such as ZnS usually has a high refractiveindex, and thus has a low output efficiency. The output efficiencydepends on the formula η_(e)=(2n²)⁻¹. Here, “n” denotes the refractiveindex of a fluorescent layer.

According to the above formula, in the case of a ZnS-based fluorescentlayer, “n” is 2.5, so only about 8% of the light is output, while mostof the light is guided between thin-film layers of the image displaydevice and disappears.

In order to overcome the above problem, there has been proposed a methodof adjusting the grain size of a fluorescent layer in anelectroluminescence display device to provoke scattering on the surfaceof a substrate made of glass, thereby increasing the output of light.This method is effective when only a thin film fluorescent layer isformed on a substrate, but is not significantly effective when anelectrode layer and an insulation layer are formed between a fluorescentlayer and a substrate.

In order to increase the light output efficiency of a fluorescent layer,a method of adjusting the partial pressure of O₂ to 200 or greater mtorrduring the formation of the fluorescent layer, so as to increase anodule dimension to 100 nm, was proposed (S. I. Jones, D Kumar, K.-G.Cho, R. Singh, and P. H. Holloway, 1999, Displays, 19, 151), and amethod for increasing a light output characteristic by four times usinga rough glass substrate was proposed (Sella, C.; Martin, J.; Charreire,Y 1982, Thin Solid Films, 90, 181).

Since light is prevented from being guided by only a particularthin-film layer, these methods have a limitation in increasing lightoutput efficiency.

U.S. Pat. No. 6,476,550 discloses an organic electroluminescence displaydevice including a thin film having a rugged surface for diffractinglight.

SUMMARY OF THE INVENTION

The present invention provides an electroluminescence display devicewhich prevents light from being guided between a substrate and thin-filmlayers, i.e., an insulation layer, a fluorescent layer, and an electrodelayer, due to a difference in refractive index there between, anddiffracts light, thereby increasing light output efficiency, and amethod of manufacturing the same.

The present invention also provides an electroluminescence displaydevice which improves the luminance of an image by increasing lightoutput efficiency so that an image can be formed through inorganicthin-film electroluminescence, and a method of manufacturing the same.

The present invention also provides an electroluminescence displaydevice, which reduces light loss using a light dispersion effect at theinterface between a high-refractive index layer and a low-refractiveindex layer, and a method of manufacturing the same.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

According to an aspect of the present invention, there is provided anelectroluminescence display device including a substrate; a corrugatedstructure formed on the substrate, wherein the corrugated structuredisperses light through diffraction and reflection; and a firstelectrode layer, a first insulation layer, a fluorescent layer, a secondinsulation layer, and a second electrode layer sequentially formed onthe substrate to follow the shape of the corrugated structure.

The corrugated structure may comprise a plurality of dots having acylindrical shape or a polygonal cone shape, and arranged in apredetermined pitch. The corrugated structure may comprise a materialhaving a substantially the same refractive index as the substrate. Thecorrugated structure may comprise transparent SiO₂ or polyimide.

A pitch between corrugating members of the corrugated structure may beλ/4 to 4λ of a wavelength of light produced from the fluorescent layer.The fluorescent layer may comprise an oxide or sulfide having arefractive index of more than 1.6 as a base material. The fluorescentlayer may have a higher refractive index than the adjacent layers.

According to another aspect of the present invention, there is providedan electroluminescence display device including a transparent substrate;a corrugated structure dispersing light through diffraction andreflection; and a first electrode layer, a first insulation layer, afluorescent layer, a second insulation layer, and a second electrodelayer sequentially formed on the substrate; wherein the corrugatedstructure is formed on the substrate or on at least one of thesequentially formed layers, and at least one of the sequentially formedlayers is formed on the corrugated structure to follow a shape of thecorrugated structure.

According to still another aspect of the present invention, there isprovided a method of manufacturing an electroluminescence displaydevice. The method includes preparing a transparent substrate; formingan insulation thin-film layer on the transparent substrate; forming aphotoresist layer on the insulation thin-film layer; patterning thephotoresist layer using a laser hologram; etching the patternedphotoresist layer; and etching the insulation thin-film layer using theetched photoresist layer, thereby forming a corrugated structure whichdisperses light through diffraction and reflection.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a cross-section of a conventional electroluminescence displaydevice;

FIG. 2 is a cross-section of an electroluminescence display deviceaccording to an embodiment of the present invention;

FIGS. 3 through 5 are perspective views of examples of a structure of anelectroluminescence display device according to embodiments of thepresent invention;

FIG. 6 is a perspective view of an example of a structure of anelectroluminescence display device according to another embodiment ofthe present invention; FIG. 7 is a cross-section of anelectroluminescence display device according to an embodiment of thepresent invention, which shows the operation of the electroluminescencedisplay device;

FIG. 8 is a graph of luminance versus applied voltage in anelectroluminescence display device having a corrugated structure and inan electroluminescence display device having an uncorrugated structure;

FIG. 9A is a cross-section of an electroluminescence display deviceaccording to still another embodiment of the present invention;

FIG. 9B is an enlarged view of the part C shown in FIG. 9A; and

FIG. 10 is a flowchart of a method of manufacturing anelectroluminescence display device according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tothe like elements throughout. The embodiments are described below inorder to explain the present invention by referring to the figures.

In an electroluminescence display device according to an embodiment ofthe present invention, a corrugated structure for dispersing lightthrough diffractive reflection is formed on the top surface of asubstrate, or at least one layer among a plurality of thin-film layersstacked, and at least one layer is corrugatedly formed on the corrugatedstructure. An embodiment of an electroluminescence display deviceaccording to the present invention is shown in FIG. 2.

Referring to FIG. 2, an electroluminescence display device 20 includes atransparent substrate 21 and a corrugated structure 30 formed on thesubstrate 21 in order to disperse light by diffracting and reflectingthe light. The electroluminescence display device 20 also includes afirst electrode layer 22, a first insulation layer 23, a fluorescentlayer 24, a second insulation layer 25, and a second electrode layer 26,which are sequentially formed on the substrate 21 having the corrugatedstructure 30, following the surface shape of the structure 30.

The first electrode layer 22 may be made of a transparent material,i.e., indium thin oxide (ITO). The fluorescent layer 24 can be formed ofa single patterned layer to emit single color light. Alternatively, thefluorescent layer 24 can be formed of red, blue, and green layers in apredetermined pattern to form a color image. In this case, the first andsecond electrode layers 22 and 26 can be formed in a matrix to beorthogonal to each other so that an electric field can be applied toboth top and bottom surfaces of each red, blue, or green layer. It ispreferable that the fluorescent layer 24 is made using a material suchas an oxide or sulfide, having a refractive index (n) of at least 1.6,as a base material. The refractive index of the fluorescent layer 24 isnot restricted to the above embodiment but is preferably greater thanthose of its adjacent layers, i.e., the first and second insulationlayers 23 and 25. The first and second insulation layers 23 and 25 canbe made of an oxide or sulfide.

As shown in FIGS. 3 through 5, the corrugated structure 30, which isformed on the top surface of the substrate 21 in order to diffract andreflect light produced from the fluorescent layer 24 for the dispersionof the light, includes a plurality of dots 31 having a cylindricalshape, the shape of a square pillar, or a tapered circumference. Theshape of the dots 31 is not restricted to the above embodiments of thepresent invention, but any shape having a predetermined pitch ispossible. In another embodiment of the present invention, as shown inFIG. 6, a plurality of recessed holes 33 are formed in a predeterminedpitch P in a substrate 21 or an insulation thin-film layer 32 toimplement a corrugated structure. The shape of the recessed holes 33 isnot restricted to a circle as shown in FIG. 6.

In the corrugated structure 30, the pitch P between the dots 31 or therecessed holes 33 has a value in a range of λ/4 through 4λ of thewavelength of light produced from the fluorescent layer 24, or in arange of 100-2400 nm. The corrugated structure 30 has a height H lessthan the height HP of the fluorescent layer 24. Substantially, thefluorescent layer 24 has a thickness of 600 nm. It is preferable to setthe height of the corrugated structure 30 taking account of thethicknesses of the fluorescent layer 24, the first electrode layer 22,and the first insulation layer 23, such that a layer can be formed onthe fluorescent layer 24, following the shape of the corrugatedstructure 30. It is preferable to set the height of the corrugatedstructure 30 to be at least 50 nm, and less than the thickness of thefluorescent layer 24.

The diameter D of the dots 31 or the recessed holes 33, and the pitch Pin the corrugated structure 30, directly influence light outputefficiency. Based on the experiments performed taking account of thisfact, it is preferable to set the diameter D to satisfy a formula0.05<2×D/P<0.5. When 2×D/P is 0.5 or greater, the dots 31 meet oneanother, almost removing corrugation. Conversely, when 2×D/P is 0.05 orless, the dots 31 are too small to be an effective corrugation.

It is preferable that the corrugated structure 30 has substantially thesame refractive index as the substrate 21, or a layer in which thecorrugated structure 30 is formed. The corrugated structure 30 can bemade of an inorganic material, such as transparent SiO₂ or polyimide, ora polymer material. However, the present invention is not restricted tothe above embodiments.

When a predetermined voltage is applied to the electroluminescencedisplay device 20 through the first and second electrode layers 22 and26, electrons pass through the first and second insulation layers 22 and25 and are injected into the fluorescent layer 24. The electronsaccelerated by a magnetic field move toward the fluorescent layer 24 tocollide with atoms, i.e., luminescence centers, to excite the electronsof the luminescence centers. When the excited electrons make thetransition to a ground state, light of a particular wavelengthcorresponding to an energy difference around the electrons is produced,which is referred to as an electroluminescence phenomenon. When an ACvoltage, or alternate positive and negative voltages having apredetermined waveform, is repeatedly applied, as manyelectroluminescence phenomena as the number of pulses occurs inalternate opposite directions so that luminescence of the fluorescentlayer 24 can be maintained.

Light produced from the fluorescent layer 24 is output through thesubstrate 21. Since the first electrode layer 22, the first insulationlayer 23, the fluorescent layer 24, the second insulation layer 25, andthe second electrode layer 26 have corrugations, due to the corrugatedstructure 30 composed of the dots 31 formed on the substrate 21 in thepredetermined pitch P, as shown in FIG. 7, light incident on aninterface at a threshold or greater angle is prevented from being guidedinward due to a refractive index difference at the interface, and istherefore dispersed. Consequently, the light is prevented from beinglost between the layers constituting the electroluminescence displaydevice.

In other words, since the refractive index of the fluorescent layer 24is relatively higher than those of the first and second electrode layers22 and 26 and the first and second insulation layers 23 and 25, light isreflected at an interface there between. However, due to the corrugationof each layer, light incident on the interface in parallel does notprogress horizontally, but is output through the substrate 21, therebyincreasing light output efficiency.

By comparing the amount of light output from an electroluminescencedisplay device having a corrugated structure according to theseembodiments of the present invention with the amount of light outputfrom an electroluminescence display device not having the corrugatedstructure, it can be proved that the present invention achieves anincrease in light output efficiency.

EXPERIMENTAL EXAMPLE 1

In this experiment, when manufacturing an electroluminescence displaydevice, a corrugated structure including dots having a cylindrical shapewas formed using SiO₂ on the top surface of a substrate. A pitch betweenthe dots was 620 nm, and the dots were 200 nm in height and 220 nm indiameter. A first electrode layer, a first insulation layer, afluorescent layer, a second insulation layer, and a second electrodelayer were corrugatedly formed on the substrate, following the shape ofthe corrugated structure.

EXPERIMENTAL EXAMPLE 2

In this experiment, when manufacturing an electroluminescence displaydevice, a corrugated structure including dots having a cylindrical shapewas formed using SiO₂ on the top surface of a substrate. A pitch betweenthe dots was 420 nm, and the dots were 200 nm in height and 220 nm indiameter. A first electrode layer, a first insulation layer, afluorescent layer, a second insulation layer, and a second electrodelayer were corrugatedly formed on the substrate, following the shape ofthe corrugated structure.

COMPARISON EXAMPLE 1

When manufacturing an electroluminescence display device without forminga corrugated structure, a first electrode layer, a first insulationlayer, a fluorescent layer, a second insulation layer, and a secondelectrode layer were flatly formed on the top surface of a substrate.

The electroluminescence of each electroluminescence display devicemanufactured in Experimental Examples 1 and 2, and Comparison Example 1,was measured using a spectrophotometer in order to estimate light outputefficiency. Luminance and light output efficiency were measured underthe condition that a variable 500 Hz sine wave voltage was applied forexcitation. In particular, they were measured at threshold voltages of40 and 60 V. As a result, with respect to the electroluminescencedisplay devices respectively manufactured in Experimental Example 1 andComparison Example 1, the graph of the applied voltage versus luminanceshown in FIG. 8 was obtained. In addition, the luminance and lightoutput efficiency, which were measured at the threshold voltages in theelectroluminescence display devices respectively manufactured inExperimental Examples 1 and 2, and Comparison Example 1, were obtainedas shown in the following table. TABLE Light output efficiency PitchDiameter Height Luminance (cd/m²) (lm/W) between of dots of dots VoltageVoltage dots (nm) (nm) (nm) (40 V) Voltage (60 V) (40 V) Voltage (60 V)Experimental 620 220 200 1780 1940 1.002 0.879 Example 1 Experimental420 220 200 1040 1190 0.670 0.587 Example 2 Comparison 688 763 0.4750.405 Example 1

As is seen from the graph shown in FIG. 8, the electroluminescencedisplay device having the corrugated structure on the substrate has 2.6or more times higher luminance than the electroluminescence displaydevice not having the corrugated structure.

In addition, as is seen from the above table, each electroluminescencedisplay device having the corrugated structure on the substrate has 2.4or more times higher luminance and 2.3 and more times higher lightoutput efficiency than the electroluminescence display device not havingthe corrugated structure.

FIGS. 9A and 9B show an electroluminescence display device according tostill another embodiment of the present invention. Referring to FIGS. 9Aand 9B, a buffer layer 41 is formed on a transparent substrate 40. Apixel area A, for forming a pixel, and a driving area B, in which a thinfilm transistor (TFT) and a capacitor are formed, are defined above thebuffer layer 41.

In the driving area B, a p-type or n-type semiconductor layer 42, whichis arranged in a predetermined pattern on the top surface of the bufferlayer 41, is covered with a gate insulation layer 43. A gate electrodelayer 44, corresponding to the semiconductor layer 42, is formed on thetop surface of the gate insulation layer 43, and is covered with a firstinsulation layer 45. In addition, a drain electrode 46 and a sourceelectrode 47 are formed on the top surface of the first insulation layer45 so that they are connected to both ends of the semiconductor layer 42through contact holes 46 a and 47 a, which are formed in the firstinsulation layer 45 and the gate insulation layer 43. A first auxiliaryelectrode 111 is formed on the top surface of the first insulation layer45 to be connected to the source electrode 47. A second auxiliaryelectrode 112 is formed to face the first auxiliary electrode 111 and tobe covered with the first insulation layer 45. The first and secondauxiliary electrodes 111 and 112 constitute a capacitor 110. A secondinsulation layer 48 is formed on the top surface of the first insulationlayer 45. A corrugated structure 30 composed of dots 31, which is thesame as described in the above embodiments, is formed on the top surfaceof the second insulation layer 48, or a portion of the second insulationlayer 48, in the pixel area A.

In the pixel area A, a third insulation layer 49 having an opening 49 ais formed on the top surface of the second insulation layer 48. A firstelectrode layer 100, electrically connected to the drain electrode 46,is corrugatedly formed on the bottom of the opening 49 a in the thirdinsulation layer 49, i.e., on the top surface of the second insulationlayer 48, on which the corrugated structure 30 is formed, following theshape of the corrugated structure 30. A fluorescent layer 50 is formedon the top surface of the first electrode layer 100. A second electrodelayer 101 is formed on the top surfaces of the fluorescent layer 50 andthe third insulation layer 49. Fourth and fifth insulation layers 51 and52 are formed between the respective first and second electrodes 100 and101 and the fluorescent layer 50. Here, the fluorescent layer 50 and theportions of the first and second electrode layers 100 and 101 and thefourth and fifth insulation layers 51 and 52, which correspond to thefluorescent layer 50, are corrugatedly formed, following the shape ofthe corrugated structure 30 formed on the second insulation layer 48. Asdescribed in the above embodiments, the refractive index of thefluorescent layer 50 is relatively higher than those of the firstthrough fifth insulation layers 45, 48, 49, 51, and 52.

Alternatively, in an electroluminescence display device using a TFT, thecorrugated structure 30 can be formed on the top surface of the firstinsulation layer 45.

In the above described electroluminescence display device, when apredetermined voltage is applied to the first and second electrodelayers 100 and 101 by a selected TFT, light is produced from thefluorescent layer 50, through electroluminescence, and is output. Here,due to the corrugations of the fluorescent layer 50, the first andsecond electrode layer 100 and 101, and the fourth and fifth insulationlayers 51 and 52, light incident at a threshold or greater angle fromthe fluorescent layer 50 on an interface therebetween is scattered sothat the light has an incident angle less than the threshold angle.Consequently, the reflective index of the interface is remarkablyreduced.

FIG. 10 is a flowchart of a method of manufacturing anelectroluminescence display device according to an embodiment of thepresent invention. Referring to FIG. 10, a substrate is prepared. Acorrugated structure for dispersing light through diffraction andreflection is formed on the top surface of the substrate. Here, if theelectroluminescence display device is designed to output light throughthe substrate, the substrate must be made of a transparent material suchas glass.

The formation of the corrugated structure can be divided into aplurality of processes, which will be described below.

After cleaning the substrate, SiO₂ is deposited to a thickness of 5000 Åusing a vacuum deposition method, thereby forming a first thin film. Inorder to mask the first thin film, Cr or Si is deposited on the firstthin film to a thickness of 500 Å, thereby forming a second thin film. Aphotoresist layer is formed on the top surface of the second thin filmand then patterned using a laser scanning or hologram method.Thereafter, the second thin film is etched. After completely etching thesecond thin film in a predetermined pattern, the first thin film isetched using the etched second thin film, thereby forming the corrugatedstructure including dots arranged in a predetermined pitch.

After forming the corrugated structure, including the dots having apredetermined size on the substrate, a first electrode layer, a firstinsulation layer, a fluorescent layer, a second insulation layer, and asecond electrode layer are sequentially formed on the substrate to havecorrugations following the shape of the corrugated structure.

A method of forming a corrugated structure on a substrate according tothe present invention is not restricted to the above embodiment. Forexample, the corrugated structure can be formed on the first or secondinsulation layer.

An electroluminescence display device according to the present inventionincludes a corrugated structure in order to form corrugations in afluorescent layer or an insulation layer, thereby reducing internallight loss and increasing light output efficiency. In particular, thepresent invention allows the light output efficiency of an inorganicelectroluminescence display device to be increased, thereby making itpossible to practically use the inorganic electroluminescence displaydevice.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A method of manufacturing an electroluminescence display device, themethod comprising: preparing a transparent substrate; forming aninsulation thin-film layer on the transparent substrate; forming aphotoresist layer on the insulation thin-film layer; patterning thephotoresist layer using a laser hologram; etching the patternedphotoresist layer; and etching the insulation thin-film layer using theetched photoresist layer, thereby forming a corrugated structure whichdisperses light through diffraction and reflection.